Seed surface morphology in some representatives of the Genus ...

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Seeds with nearly smooth cuticle were most common,
some specimens had a helical or granular surface (Fig. 3:
4; Fig. 4: 2).
Cross-sections clearly show that the exotesta anticlinal
walls are raised, S-shaped, and they form folds that
appear to thicken, while the periclinal wall is concave
and nearly aligned with the inner wall (Fig. 1: 3, 4).
Thus, the study of the seed characters in seven taxa of
predominantly Far-Eastern Rhodiola section Rhodiola
representatives revealed a heterogeneous seed surface
sculpturing in the section. This observation contradicts
data on other Crassulaceae genera, Hylotelephium,
Aizopsis and Orostachys, traditionally considered to be
closely related to Rhodiola. It has been shown that their
intrageneric taxa (section and series) are characterized
by a uniform set of seed coat features that could be used
to discriminate these entities (Gontcharova, 1999; Hart
and Berendsen, 1980; Knapp, 1994; Koldaeva and
Gontcharova, 2005). Here we report three distinct types
of seed coat sculpture, each characterized by a specific
anatomical structure (Table 1, Figs. 1–4). Variability of
this character in the section under discussion is not in
agreement with the stability of this character in many
other plant genera (Barthlott, 1984; Esau, 1977). This
ARTICLE IN PRESS
S.B. Gontcharova et al. / Flora 204 (2009) 17–24
Fig. 4. Scanning electron micrographs of Rhodiola seeds. 1–6 tenuicostate surface: Rh. rosea subsp. rosea (1, 2), Rh. integrifolia
(3, 4); R. rosea subsp. arctica (5); Rh. rosea subsp. sachalinensis (6).
may be indicative of the section’s artificial nature.
Indeed, a recent molecular study on phylogenetic
relationships in Sedoideae could not define Rhodiola
sections and series as monophyletic entities (Mayuzumi
and Ohba, 2004). The current taxonomic structure of
the genus remains therefore in question, and taxon
sampling in both molecular and seed micromorphology
analyses is still too limited to draw any conclusion from
seed coat features in order to characterize Rhodiola
supraspecific taxa.
It is also evident that the significance of seed coat
sculpturing at the lower taxonomic levels is not uniform
in the genus Rhodiola. The characteristics of seed coat
sculpturing are nearly identical in the three taxa that are
characterized by a colliculate seed surface, Rh. alsia,
Rh. angusta and Rh. ishidae, with the consequence of
little value in species distinction. The close relationship
of these taxa, based on their similar vegetative
morphology, has already been stressed by Ohba
(1981). The common seed coat sculpture type and the
presence of large wing-like projection or folds in all of
them emphasize this as well. The common seed coat
features are known only in these three species, differentiating
them from other members of the sect.